The present invention relates to a process for the production of ethylenically unsaturated carboxylic acids or esters, particularly α, β unsaturated carboxylic acids or esters, more particularly acrylic acids or esters such as (alk)acrylic acids or alkyl (alk)acrylates particularly (meth)acrylic acid or alkyl (meth)acrylates by the condensation of carboxylic acid or esters with formaldehyde or a source thereof such as dimethoxymethane in the presence of catalysts, in particular, but not exclusively, a process for the production of (meth) acrylic acid or alkyl esters thereof, for example, methyl methacrylate, by the condensation of propionic acid or alkyl esters thereof with formaldehyde or a source thereof such as dimethoxymethane in the presence of such a catalyst system. The invention is particularly relevant to the production of methacrylic acid (MAA) and methyl methacrylate (MMA).
Such acids or esters may be made by reacting an alkanoic acid (or ester) of the formula R3—CH2—COOR4, where R3 and R4 are each, independently, a suitable substituent known in the art of acrylic compounds such as hydrogen or an alkyl group, especially a lower alkyl group containing, for example, 1-4 carbon atoms, with a suitable methylene source such as formaldehyde. Thus, for instance, methacrylic acid or alkyl esters thereof, especially methyl methacrylate, may be made by the catalytic reaction of propionic acid, or the corresponding alkyl ester, e. g. methyl propionate, with formaldehyde as a methylene source in accordance with the reaction sequence 1.R3—CH2—COOR4+HCHO→R3—CH(CH2OH)—COOR4 andR3—CH(CH2OH)—COOR4→R3—C(:CH2)—COOR4+H2O  Sequence 1
An example of reaction sequence 1 is reaction sequence 2CH3—CH2—COOR4+HCHO→CH3—CH(CH2OH)—COOR4 CH3—CH(CH2OH)—COOR4→CH3—C(:CH2)—COOR4+H2O  Sequence 2
A further reaction sequence is one which uses an acetalR3—CH2—COOR4+R′OCH2OR″→R3—C(:CH2)—COOR4+R′OH+R′OH  Sequence 3
A theoretical example of reaction sequence 3 is reaction sequence 4 which uses dimethoxymethaneCH3—CH2—COOR4+CH3OCH2OCH3→CH3—C(:CH2)—COOR4+2 CH3OH  Sequence 4
The use of dimethoxymethane thus theoretically gives an anhydrous system which avoids the difficulty of subsequent water separation and/or subsequent product hydrolysis. In addition, the use of dimethoxymethane avoids the use of free formaldehyde but nevertheless acts in a general sense as a source of formaldehyde. The absence of water and free formaldehyde could greatly simplify the separation of MMA from the product stream.
However, in practice, Sequence 4 is problematic because methanol dehydrates to dimethyl ether and water. In addition, dimethoxymethane decomposes under catalytic conditions to dimethylether and formaldehyde. Any water formed in these reactions can hydrolyse the ester feedstock or product to its corresponding acid which may be undesirable.
U.S. Pat. No. 4,560,790 describes the production of α, β unsaturated carboxylic acids and esters by the condensation of methylal(dimethoxymethane) with a carboxylic acid or ester using a catalyst of general formula M1/M2/P/O wherein M1 is a group IIIb metal, preferably aluminium, and M2 is a group IVb metal, preferably silicon.
As mentioned above, a known production method for MMA is the catalytic conversion of methyl propionate (MEP) to MMA using formaldehyde. A suitable catalyst for this is a caesium catalyst on a support, for instance, silica.
U.S. Pat. No. 4,118,588 discloses the production of methyl methacrylate and methacrylic acid by reacting propionic acid or methyl propionate with dimethoxymethane in the presence of catalysts based on the phosphates and/or silicates of magnesium, calcium, aluminium, zirconium, thorium and/or titanium and also in the presence of 0 to 0.5 moles of water per mole of the acetal. The preferred phosphates are aluminium, zirconium, thorium and titanium. The catalysts generally include an oxide modifier to improve the catalytic activity. Magnesium phosphate is not exemplified and calcium phosphate is not exemplified alone but one example with an oxide modifier is provided. The results are poor compared with the other phosphates, particularly aluminium.
Gupta et al in the Beilstein Journal of Organic Chemistry 2009, 5, No. 68 disclose the Knoevenagel condensation between aromatic aldehydes and malononitrile, ethyl cyanoacetate or malonic acid with hydroxyapatite supported caesium carbonate in water. However, the condensation with malonic acid resulted in decarboxylation.
Calcium hydroxyapatite exists in a number of crystalline forms. In addition, amorphous precursors of Hydroxyapatite, with calcium:phosphorus ratios which are similar to those for crystalline forms are disclosed. These can convert to crystalline Hydroxyapatite either by a physical or chemical treatment. The crystalline forms are generally divided into two types:—rods and plates but crystalline nano-spheres are also known. These three crystal forms are well documented in the scientific literature. The typical natural rod-like and plate-like crystal forms of hydroxyapatite are disclosed in many documents for example in J Mater Chem 2004, 14, 2277, Rosanna Gonzalez-McQuire et al; Particuology 2009, 7, 466, Padmanabhan et al; Chemical Physics Letters 2004, 396, 429, Liu et al; Biomaterials 2007, 28, 2275, Chen et al; and Journal of the Japan Petroleum Institute 2009, 52, 51, Tsuchida et al.
Hydroxyapatite in the rod-like crystal form may develop structures such as bowknot-like or flower-like structures (Chemical Physics Letters 2004, 396, 429 by Liu).
The conditions for producing the various crystal forms of calcium hydroxyapatite are also well documented (J Mater Chem 2004, 14, 2277, Rosanna Gonzalez-McQuire et al; Particuology 2009, 7, 466, Padmanabhan et al; Chemical Physics Letters 2004, 396, 429, Liu et al; Biomaterials 2007, 28, 2275, Chen et al; Journal of the Japan Petroleum Institute 2009, 52, 51, Tsuchida et al; and J Phys Chem B 2007, 111, 13410, Tao et al). In addition, conversion of nano-spheres into rod-like and sheet-like structures has been disclosed by Tao et al (J Phys Chem B 2007, 111, 13410).
Specifically, methods for producing hydroxyapatite rods are well documented in the literature. Hydroxyapatite rods have been successfully synthesized using hydrothermal (Zhang et al., Journal of Crystal Growth, 2007, 308, 133-140), wet chemical (Materials Chemistry and Physics, 2004, 86, 69-73, Liu et al), ultrasonic spray pyrolysis (Materials Science and Engineering A, 2007, 449-451,821-824, An et al) and sol-gel routes (Particuology 2009, 7, 466, Padmanabhan et al).
Most of the interest in the natural crystal forms of hydroxyapatite relates to its use or application in the study of biomedical applications due to its similarity to human bone. Few of the studies of morphological effects relate to industrial catalytic applications of hydroxyapatite.
Crystalline spheres or nano-spheres or amorphous calcium phosphates with calcium:phosphorus ratios similar to crystalline hydroxapatites in the form of spheres and nano-spheres are also well documented in the literature and are generally favoured by manufacturers (J Phys Chem B 2007, 111, 13410 Tao et al). Sometimes a crystalline core is encapsulated by an amorphous shell to create spheres. However, amorphous spheres can form initially followed by subsequent crystallisation as disclosed by Kandori et al (Polyhedron 2009, 28, 3036). Catalytic applications of hydroxyapatite are known but no mention of crystallinity or particular crystal forms is disclosed therein. Due to the wide availability of nano-spherical amorphous precursors or crystal forms of hydroxyapatite it can be assumed that the catalytic applications relate to this common amorphous or nano-spherical form unless otherwise mentioned.
Surprisingly, it has now been found that particular metal phosphates of a particular crystal form are remarkably selective catalysts for the production of α, β ethylenically unsaturated carboxylic acid or esters by condensation of the corresponding acid or ester with a methylene source such as formaldehyde or dimethoxymethane providing high selectivity and low dimethylether (DME) production. In particular, the catalysts are particularly suited to the production of α, β ethylenically unsaturated carboxylic esters because they produce little water in such reactions.